Ball spline having reinforcement bush part

A ball spline (100) having reinforcement bush parts, comprising:

• a rod-shaped spline shaft (110) having two concave track grooves (111) formed to extend in the longitudinal direction thereof;
• a nut part in which the spline shaft (110) is inserted so that the nut part can move in the longitudinal direction of the spline shaft;
• and
• a plurality of balls (150) provided between the nut part and the spline shaft (110) so as to circulate,
• wherein the nut part includes a nut (120) which is a hollow body into which the spline shaft (110) is inserted, and retainers (130), which are hollow bodies provided at both sides of the nut (120) in the longitudinal direction of the nut (120) and into which the spline shaft (110) is inserted;
• wherein the nut part is provided with two spline parts;
• wherein each of the spline parts includes:
• a concave load track groove (125), which is formed in the longitudinal direction on an inner surface part (121) of the nut (120) so as to face a corresponding one of the track grooves (111),
• a no-load circulation part (127) formed in parallel to the load track groove (125) in the longitudinal direction, and
• a curved concave spline circulation groove (133), which is formed in each of the retainers (130) and has one side facing the end portion of the load track groove (125) and the other side facing the end portion of the no-load circulation part (127);
• wherein some of the balls (150) are diverted in the spline circulation grooves (133) of the retainers (130) which are provided on both sides in the longitudinal direction, and circulate and move between the track grooves (111) and the load track grooves (125) and along the no-load circulation parts (127);
• wherein the nut part further includes two reinforcement bush parts in parallel with the spline parts;
• wherein the remainder of the balls (150) roll in contact with the outer surface of the spline shaft (110) and circulate and move along the reinforcement bush parts;
• wherein the two track grooves (111) are provided at positions opposite to each other, and the reinforcement bush parts are provided alternatively with the spline parts so that each of the reinforcement bush parts is positioned between the spline parts, with a phase of 90° with respect to each of the spline parts;
• and
• wherein a gap between a bush track groove (125-1) and the outer surface of the spline shaft (110) is formed to be larger than a gap between each of the load track grooves (125) and each of the track grooves (111), so that a preload acting on the balls (150) between the outer surface of the spline shaft (110) and the bush track groove (125-1) is smaller than a preload acting on the balls (150) between each of the track grooves (111) and each of the load track grooves (125).